In the context of global alterations of the biogeochemical cycles, mountain freshwater ecosystems
(e.g., rivers and lakes) are considered true sentinels of change due to their high connectivity with
terrestrial ecosystems. These ecosystems usually are oligotrophic and subjected to low external
loads of nutrients. Among the biogeochemical cycles, that of nitrogen (N) is one currently under
greater alteration at a global scale. This acceleration is due to the increased availability of reactive
nitrogen (Nr) in the environment which has at least doubled since preindustrial times because of
human activities. Wet deposition of Nr, mainly in the form of nitrate, is a major source of N input
to sediments and the water column of mountain freshwater ecosystem rivers and lakes. Nitrogen
forms may then be exported downstream and affect other freshwater ecosystems.
Within the N cycle, dissimilatory reduction of nitrate to ammonium (DNRA), chemoautotrophic
denitrification via sulfur or iron oxidation, anammox, and denitrification are the only known
biological processes involved in removal of Nr species in the biosphere. Other processes such as
N-fixation and nitrification also contribute to N transformations in freshwater ecosystems. Among
these processes, the denitrification pathway is highly relevant to the biosphere availability of Nr
because it is the primary process of removing nitrous oxide (N2O), a potent greenhouse gas that is
the major cause of stratospheric ozone depletion. Nitrogen transformation dynamics in freshwater
ecosystems can be episodic and spatially heterogeneous. Such variation occurs due to fluctuations
in resources (e.g., nitrate) and conditions (e.g., temperature and oxygen availability). However,
few studies have examined the relevance of N-transformations in freshwater ecosystems, and
whether different N-transformation processes can simultaneously co-occur remains largely
unknown.
The importance of microbial activity to ecosystem function in aquatic ecosystems is well
established, but microbial diversity has been less frequently addressed. Despite the fundamental
role of microbial communities in N cycling, the relationships between N transformations to
microbial abundance and diversity and their controlling factors are largely unknown in freshwater
ecosystems. For example, few studies have examined the abundance and diversity of key N-functional genes involved in N-fixation, nitrification, denitrification, DNRA and anammox
pathways not only in high mountain lakes or rivers but also in their downstream fluvial systems.
In addition, whether some microbial communities and/or specific microorganisms could be used
as potential biological indicators of climate change and/or anthropogenic contamination in these
ecosystems remains to be elucidated.
This special issue welcomes original research, reviews, mini-reviews, opinions, methods, hypothesis and theory, and perspective. The directions include but are not limited to the following:
- Identification of abiotic and biotic factors explaining changes in N-cycling and N-transformation rates in freshwater ecosystems
- Characterization of N-functional pathways contributing to N2O emissions
- Effect of altitudinal gradients and inter-annual variability on N-cycling and microbial communities
In the context of global alterations of the biogeochemical cycles, mountain freshwater ecosystems
(e.g., rivers and lakes) are considered true sentinels of change due to their high connectivity with
terrestrial ecosystems. These ecosystems usually are oligotrophic and subjected to low external
loads of nutrients. Among the biogeochemical cycles, that of nitrogen (N) is one currently under
greater alteration at a global scale. This acceleration is due to the increased availability of reactive
nitrogen (Nr) in the environment which has at least doubled since preindustrial times because of
human activities. Wet deposition of Nr, mainly in the form of nitrate, is a major source of N input
to sediments and the water column of mountain freshwater ecosystem rivers and lakes. Nitrogen
forms may then be exported downstream and affect other freshwater ecosystems.
Within the N cycle, dissimilatory reduction of nitrate to ammonium (DNRA), chemoautotrophic
denitrification via sulfur or iron oxidation, anammox, and denitrification are the only known
biological processes involved in removal of Nr species in the biosphere. Other processes such as
N-fixation and nitrification also contribute to N transformations in freshwater ecosystems. Among
these processes, the denitrification pathway is highly relevant to the biosphere availability of Nr
because it is the primary process of removing nitrous oxide (N2O), a potent greenhouse gas that is
the major cause of stratospheric ozone depletion. Nitrogen transformation dynamics in freshwater
ecosystems can be episodic and spatially heterogeneous. Such variation occurs due to fluctuations
in resources (e.g., nitrate) and conditions (e.g., temperature and oxygen availability). However,
few studies have examined the relevance of N-transformations in freshwater ecosystems, and
whether different N-transformation processes can simultaneously co-occur remains largely
unknown.
The importance of microbial activity to ecosystem function in aquatic ecosystems is well
established, but microbial diversity has been less frequently addressed. Despite the fundamental
role of microbial communities in N cycling, the relationships between N transformations to
microbial abundance and diversity and their controlling factors are largely unknown in freshwater
ecosystems. For example, few studies have examined the abundance and diversity of key N-functional genes involved in N-fixation, nitrification, denitrification, DNRA and anammox
pathways not only in high mountain lakes or rivers but also in their downstream fluvial systems.
In addition, whether some microbial communities and/or specific microorganisms could be used
as potential biological indicators of climate change and/or anthropogenic contamination in these
ecosystems remains to be elucidated.
This special issue welcomes original research, reviews, mini-reviews, opinions, methods, hypothesis and theory, and perspective. The directions include but are not limited to the following:
- Identification of abiotic and biotic factors explaining changes in N-cycling and N-transformation rates in freshwater ecosystems
- Characterization of N-functional pathways contributing to N2O emissions
- Effect of altitudinal gradients and inter-annual variability on N-cycling and microbial communities